Colorado passed a law last spring that presumes you’re too high to drive if you have 5 nanograms or more of THC per milliliter of blood, even though many experts say there is insufficient evidence to tie that level of THC to impaired driving.

“We don’t have a consensus as to what levels of THC are consistently correlated with behavioural impairment,” Paul Armentano, deputy director for the pot advocacy group NORML, told me. But, he added, “Marijuana policy has never been driven by science in this country.”

It’s never been legal to drive while you’re impaired by any drug in Colorado, but this is the first time there’s a presumption that a certain level of THC in your blood means you’re high. Since the controversial new limits passed, Denver criminal defence attorney Sean McAllister told me he’s seen an uptick in marijuana-related driving arrests.

Medical pot users also fear getting stopped now since they may have THC levels above the legal limit but don’t feel too high to drive. Teri Robnett — a patient advocate who also has a pot prescription for her Fibromyalgia — told me she’s often over the THC limit because the medical marijuana she takes metabolizes slowly.

“I can pretty much assume that I will always be above five nanograms in my blood,” she said, “but I have no impairment.”

The Rise Of Drug-Impaired Driving Laws

Every state in America has a legal driving limit for blood alcohol levels, as well as some kind of law regulating drug use and driving. These laws are known as DUID laws, which stands for driving under the influence of drugs. The idea behind these laws is that being high on the road may be dangerous.

Some states like New York have effect-based DUID laws, meaning that you can get in trouble if you’re visibly impaired while driving and provably high on drugs that made you impaired.

Other states have per se drugged driving laws, meaning you can get a DUID if you have a certain level of drugs in your system. Several states have enacted per se laws in recent months, including Washington state and Colorado. Still other states like Arizona and, more recently Oklahoma, have “zero tolerance” per se laws. That means you’re toast if you’re caught driving with any amount of illegal drugs in your system.

While Colorado just legalized recreational pot in Nov. 2012, medical marijuana there has been legal there since 2000, and medical-marijuana activists balked at a driving limit for pot in the state when the measure was previously defeated.

Before the Colorado DUID bill failed in May 2012, medical marijuana activists argued that 5 nanograms was too low and that effectively sober people would end up getting arrested, the Denver Post reported.

“There needs to be a way to know whether a medical patient has the 5 nanograms in their system so they can know whether they can get behind the wheel,” Debbie Olander, a union spokesperson, told the Denver Post. “With alcohol you can.”

THC limits in Colorado eventually passed after the state legalized recreational pot.

Unlike in Washington, though, Colorado’s new law lets drivers try to submit evidence in court showing they weren’t impaired even though their THC blood levels were above the legal limit. That standard is known as “permissive inference.” But you’d have to have a good lawyer to get out of a DUID with 5 nanograms in your blood, even if you were cool to drive.

Marijuana advocates say there simply isn’t enough evidence to link certain THC levels to impaired driving, even though many states have tied a specific number to impairment.

To be sure, there is some evidence that it’s not a great idea to drive when you’re high.

“We know that when people smoke marijuana they lose some of their peripheral vision,” Dr. Marilyn Huestis, a senior investigator at the National Institute on Drug Abuse’s Intramural Research Program, told Popular Science Magazine. “We know it affects the passage of time, or the idea of how rapidly time is passing. It affects balance. And one of the most interesting areas it affects is the prefrontal cortex.”

That, in turn, can affect our ability to make decisions, Huestis told Popular Science’s Clay Dillow. Marijuana can also make it hard to multitask. In several studies, marijuana has also interfered with drivers’ ability to hold the vehicle in the middle of the traffic lane, Dillow reported.

One widely cited experiment by a Washington TV station asked volunteers to get high on pot and then try driving on a course with a safety instructor. One drove too slowly, while another almost hit the station’s photographer.

Despite these signs that driving high might be bad, even the federal government says there’s a lack of evidence that ties a certain level of THC with a certain degree of impairment. In other words, some people may be able to drive perfectly well at a certain level of THC intoxication, while others may be impaired. Here’s what the National Highway Traffic Administration says on its website:

It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects … It is inadvisable to try and predict effects based on blood THC concentrations alone.

Apparently, Colorado’s lawmakers didn’t this advice, even though there’s also no evidence that “per se” pot laws lower traffic fatalities, according to NORML’s Paul Armentano.

For now, medical users who might have too much THC in their blood should probably try to be inconspicuous on the road.

“They drive under tremendous risk,” Colorado lawyer Rachel Gillette told me. “I always say make sure your taillights are good. Don’t swerve or go out of the lines. Make sure they don’t have any reason to pull you over in the first place.”